High surface-area materials with nanoscale dimensions are of special interest in applications where active site mediated chemical reactions play an important role, such as catalytic applications where a high contact area between reactants and catalyst is necessary in order to achieve high yield in a cost-effective manner.
Mesoporous materials offer extremely high contact areas by having porosity by means of nanoporous frameworks. The ordered pore structure and defined pore connectivity of the mesoporous materials make them suitable as nano-reactors to confine growth of nano-materials. The challenge to produce materials with controlled size and shape in nanometer scale has been much advanced the mesoporous and related materials.
Major fields of use for nanoporous frameworks are as catalysts. The catalytic applications include applications such as pollution control, water purification, air filtration, mercury remediation as well as their use as catalyst for synthetic purposes such as petroleum refinement, acid catalyst, redox reaction catalyst.
Nanoporous materials can also be used as sensors to detect gas molecules absorbed in the channels.
Another area for the use of nanoporous frameworks is as rechargeable batteries and in fuel cells. The high contact area of the nanoporous framework allows for fast interaction of the active sites with the surrounding media.
The main requirements for the active materials in nanoporous frameworks to be used for catalyst and the like applications are:                High specific surface area        Controlled physicochemical and structural properties        Controlled composition which also allows for the formation of X doped with Y metal oxide, or mixtures of different metal oxides or metals.        Synthesis procedures allowing for homogenous incorporation of metal oxide into the internal and external surfaces of nanoporous framework        Cheap and reliable synthesis procedures        
Since the discovery of mesoporous materials a great deal of attention has focused on finding practical ways of producing such materials. However, despite the high contact area given by the porous framework researchers still have a problem to produce a material giving high catalytic yields at reasonable cost level. Another problem is that poor hydrothermal stability of the mesoporous silicates produced makes them unsuitable for many catalyst applications. A method for incorporating metal active sites into mesoporous materials is reported by A Corma et al in Chem. Commun, page 1899, 1998. This method relies on costly post-synthetic treatments of the amorphous silica framework.
Transition metal oxide doped silica films have been reported by Huesing et al. in Applied Catalysis A: General 254, page 297-310, 2003. This method although potentially universal does not allow for the formation of metal oxide particles within the voids of the pores as evident from the EXFAS, FTIR and catalytic data presented which suggest that at higher loadings of the metal oxide (M:Si=1:5, where M is Ti) the catalyst sites are less active.
Other attempts include direct formation of metal oxide mesoporous materials by a generalized synthetic route. These processes usually require expensive alkoxide reactants as metal oxide precursors and yields materials where the loading of metal oxide can not exceed the amount of silica in the material.
CVD methods have also been used for incorporation of metallic centers into the channels of mesoporous solids. A problem with these methods are that the pores get blocked which reduces the surface area of these materials and thus their use as a catalyst.
US 2004/0047798 shows an electrode material for an electrical double-layer capacitor consisting of a carbon material having 2-20 nm of mesopore and metal oxides deposited in the pores. The mesoporous carbon material is produced by first preparing an inorganic template/carbon precursor composite in which the inorganic template particles are dispersed in the carbon precursor solution. Secondly the inorganic template/carbon precursor is prepared through carbonization of the carbon precursors surrounding the inorganic templates by heating the inorganic/carbon precursors at 600 to 1500° C. and thirdly to etch the inorganic template/carbon composite with base or acid to remove the inorganic template followed by drying. A post-synthetic deposition of metal oxides for the formation of a carbon/metal oxide composite material is described. A problem with post-synthetic depositions of metal oxide particles is that the incorporation of metal oxide centers will not be homogenous and it is very difficult to control the growth of the metal oxide particles. The internal surface area of the porous material will therefore be reduced and the catalytic activity of such materials is not optimized.
WO 03076702 discloses a method for producing hollow fibers for producing meso-and nanotubes. The method includes the steps of preparing a porous template, add a liquid mixture of the desired tube material, such as a metal precursor, and at least one polymer to the template material in a such a way that the pore surfaces are wetted by the liquid but the pores are not completely filled, solidify the liquid and at least partly remove the template material. WO03064081 (JP2003221601) shows a dispersion and adsorption method upon an already formed non-porous particle. The porosity of the material will depend on the size of the silica particle used. When the silica particles are dissolved the remaining material is in metal form and could not be used in metal oxide form. The system is proposed for providing a porous nano-structured body of a noble metal.
US20040118698 shows a process for preparing of a metal-containing nanostructured film comprising the steps of (a) electrodepositing a metallic composition within the pores of a mesoporous silica film to form a metal-containing silica nanocomposite, (b) annealing the nanocomposite at a temperature in the range of about 25 to 70% of the melting temperature of the metallic composition and (c) removing the silica from the nanocomposite to provide a self-supporting metal-containing nanostructured film.
This method is similar to CVD but here the metal oxide is electrodeposited. Also, this method involves the use of a costly mold prior to the incorporation step.